The present disclosure relates to a storage apparatus (or a memory) used for recording data in each of storage devices composing the storage apparatus in accordance with a spin-torque magnetization inverting method.
With rapid development of various kinds of information equipment ranging from a mobile terminal to a high-performance server, devices composing a memory and a logic circuit which are employed in such equipment are also requisite to display high performance typically by being highly integrated and being capable of operating at a high speed and a low power consumption.
In particular, the progress of a semiconductor nonvolatile memory is remarkable. For example, the popularization of a flash memory serving as a large-capacity file memory is advancing at a pace driving out the hard-disk drive.
In the mean time, aimed at code storage applications and further development toward a working memory, the development of a semiconductor nonvolatile memory supposed to replace memories generally used nowadays is making progress. Typical examples of the memories generally used nowadays are the NOR flash memory and the DRAM whereas typical examples of the semiconductor nonvolatile memory supposed to replace the memories generally used nowadays are a FeRAM (Ferroelectric Random Access Memory), a MRAM (Magnetic Random Access Memory) and a PCRAM (Phase Change Random Access Memory). Some of the semiconductor nonvolatile memories supposed to replace the memories generally used nowadays have been put to practical use.
The MRAM which is a typical semiconductor nonvolatile memory stores data as a direction of magnetization of a magnetic substance composing the MRAM. Thus, stored data can be updated at a high speed. In addition, data stored at a storage location can be updated an infinite number of times. To put it concretely, data stored at a storage location can be updated at 1015 or more times. The MRAM is already used in fields including industrial automation and avionic equipment.
Since the MRAM operates at a high speed and with a high degree of reliability, the development of the MRAM toward a code storage memory and/or a working memory is expected in the future.
However, the MRAM raises problems when efforts are made to lower the power consumption of the MRAM and increase its storage capacity.
These problems are intrinsic problems caused by the recording principle of the MRAM. In accordance with a recording method based on the recording principle of the MRAM, magnetization is inverted by a field generated by a current flowing through a wire.
As one method for solving the problems described above, recording methods (that is, magnetization inverting methods) not relying on such a current-generated field are studied. The recording methods include a spin-torque magnetization inverting method which is a subject of extensive and intensive research. For more information on the spin-torque magnetization inverting method, the reader is advised to refer to documents such as U.S. Pat. No. 5,695,864 and Japanese Patent Laid-Open No. 2003-17782.
Much like the MRAM, a storage device operating in accordance with the spin-torque magnetization inverting method is constructed from an MTJ (Magnetic Tunnel Junction).
The MTJ of the storage device includes a fixed-magnetization layer and a storage layer. The fixed-magnetization layer is a layer magnetized in a certain fixed direction whereas the storage layer is a layer magnetized not in a fixed direction. A tunnel junction is created by providing a tunnel insulation layer between the fixed-magnetization layer and the storage layer.
Data of 0 or 1 is read out from the MTJ by the so-called tunnel magnetic resistance effect in which the resistance of the MTJ changes in accordance with an angle formed by the fixed direction of the magnetization of the fixed-magnetization layer and the direction of the magnetization of the storage layer.
In a write operation, on the other hand, when spin polarized ions passing through the fixed-magnetization layer enter the storage layer, the electrons apply torques to the magnetic layer and, if a current having a magnitude at least equal to a threshold value determined in advance flows due to the torques, the direction of the magnetization of the storage layer is inverted.
Data of 0 or 1 to be written into a storage device in a write operation is selected by changing the polarity of the current flowing to the storage device.
In the case of a storage device of a scale of about 0.1 μm, the absolute value of a current for inverting the direction of the magnetization of the storage layer of the storage device is not greater than 1 mA.
In addition, this value of the current decreases proportionally to the volume of the storage device, making scaling possible.
On top of that, it is not necessary to provide a word line for producing a current-generated magnetic field required by the MRAM to serve as a field for recording data. Thus, this storage device offers a merit of a simple cell structure.
In the following description, an MRAM adopting the spin-torque inversion method is referred to as an ST-MRAM (Spin Torque-Magnetic Random Access Memory).
As a nonvolatile memory allowing the power consumption thereof to be decreased and the storage capacity thereof to be increased while sustaining the merits offered by the MRAM as they are, the ST-MRAM is much expected. It is to be noted that the merits offered by the MRAM are the high operation speed and the infinite number of allowable updating operations.